Your best option is probably a site like Craigslist, though it seems unlikely that you'll find one offered for free; trumpets are valuable enough that anyone who has a working one is probably going to want to recover at least part of their investment. (For that matter, even one that doesn't work is probably worth at least a few bucks as scrap metal.)
Any music shop that sells trumpets.
Most violin shops do appraisals.
Free printable sheet music for the trumpet can be found on the following websites: MakingMusicFun.net - Free trumpet sheet music for beginning-intermediate students. The quality of the arrangements is very high. DrFezell.com - Free trumpet sheet music for intermediate-advanced students. The quality of the arrangements is very high. 8Notes.com - Free trumpet sheet music for beginning-advanced students. The free sheet music is available in .gif format. If you subscribe you can download a higher quality PDF. See Links Below
I am guessing, at a music store
Nowhere.
Trumpet
their is a G trumpet. F trumpet B flat trumpet and a Piccolo trumpet being the smallest trumpet and an octive higher than a regular trumpet.
trumpetstuff.com/songanimal/free
Legally, nowhere.
The internet
From the interwebs
You cant
Free printable sheet music for the trumpet can be found on the following websites: MakingMusicFun.net - Free trumpet sheet music for beginning-intermediate students. The quality of the arrangements is very high. DrFezell.com - Free trumpet sheet music for intermediate-advanced students. The quality of the arrangements is very high. 8Notes.com - Free trumpet sheet music for beginning-advanced students. The free sheet music is available in .gif format. If you subscribe you can download a higher quality PDF. See Links Below
on blodidy.com
Internet
I am guessing, at a music store
The answer is: you can't. You'll need to figure out the notes, write them down, practice playing along with the song and that's the only way you will have the free mariachi trumpet music you requested.
Nowhere.
Nowhere